Operation Of Transport Refrigeration Systems To Prevent Engine Stall And Overload

ABSTRACT

The performance of a transport refrigeration system having a transport refrigeration unit powered by a diesel engine is optimized by matching a capacity output of the transport refrigeration unit to an available shaft power of the diesel engine.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.13/876,270, filed on Mar. 27, 2013, which is a 371 of internationalapplication PCT/US2011/052265 filed on Sep. 20, 2011 which claimspriority to U.S. provisional application 61/387,177 filed on Sep. 28,2010, and all the benefits accruing therefrom under 35 U.S.C. §119, thecontents of which in its entirety are herein incorporated by reference.

TECHNICAL FIELD

This invention relates generally to the operation of a transportrefrigeration system and, more particularly, to maintaining coolingperformance of a transport refrigeration system while preventing enginestalls as well as overload of the engine.

BACKGROUND

Fruits, vegetables and other perishable items, including meat, poultryand fish, fresh or frozen, are commonly transported in the cargo box ofa truck or trailer, or in an intermodal container. Accordingly, it iscustomarily to provide a transport refrigeration system in operativeassociation with the cargo box for cooling the atmosphere within thecargo box. The transport refrigeration system includes a refrigerantvapor compression system, also referred to as a transport refrigerationunit, and an on-board power unit. The refrigerant vapor compressionsystem typically includes a compressor, a condenser, an expansion deviceand an evaporator serially connected by refrigerant lines in a closedrefrigerant circuit in accord with known refrigerant vapor compressioncycles. The power unit includes an engine, typically diesel powered.

In many truck/trailer transport refrigeration systems, the compressor ofthe transport refrigeration unit is driven by the engine shaft eitherthrough a belt drive or by mechanical shaft-to-shaft link. Morerecently, all electric transport refrigeration systems have beendeveloped for truck/trailer applications wherein the engine drives anon-board generator for generating sufficient electrical power to drivean electric motor operatively associated with the compressor of thetransport refrigeration unit. For example, U.S. Pat. No. 6,223,546,assigned to Carrier Corporation, the same assignee to which thisapplication is subject to assignment, the entire disclosure of which isincorporated herein by reference in its entirety, discloses anelectrically powered transport refrigeration unit powered by an enginedriven synchronous generator capable of producing sufficient power tooperate the compressor drive motor and at least one fan motor. Withrespect to intermodal containers, clip-on power units, commonly referredto as generator sets or gensets, are available for mounting to theintermodal container, typically when the container is being transportedby road or rail, to provide electrical power for operating thecompressor drive motor of the transport refrigeration unit associatedwith the container. The genset includes a diesel engine and a generatordriven by the diesel engine.

During transport of such perishable items the temperature within thecargo box of the truck, trailer or container must be maintained withinstrict temperature limits associated with the particular items beingtransported, regardless of potentially severe operating conditionsimposed by the local environment in which the system is operating. Forexample when the transport refrigeration system is operated at highambient temperatures and/or high altitude operation, the power demandedby the refrigeration unit at high cooling capacity demand may exceed thelimited shaft power available from the engine, raising the potential foran engine stall or engine overload. In the event of an engine stall orengine overload, the loss of power from the generator will result in anundesired shutdown of the refrigeration unit.

In conventional transport refrigeration systems, the control system isopen loop in that the system controller is unaware of the actualoperating engine load. Rather, the transport refrigeration systemcontroller uses algorithms that include safety margins to limit theengine shaft power demand in an attempt to prevent overload of theengine. However, at times, such as under aggravated service conditionsand during transient operations, a lost in refrigeration unitperformance and engine stalls or overload can still occur. A need existsfor controlling the operation of the refrigeration unit in response toactual engine operating conditions so as to avoid engine stall or engineoverload.

SUMMARY

In an aspect, a method is provided for controlling the performance of atransport refrigeration system having a transport refrigeration unitpowered by an engine, comprising matching a capacity output of thetransport refrigeration unit to an available shaft power of the engine;the method further comprising: monitoring an operating engine load ofthe engine; monitoring an operating engine speed of the engine;operating the transport refrigeration system at the capacity outputnecessary to meet a current refrigeration demand load so as long as boththe operating engine load of the engine is not at or above apredetermined maximum value and the operating speed of the engine is notbelow a predetermined engine operating speed; and reducing the coolingcapacity output of the refrigeration unit when the operating engine loadof the engine is at or above the predetermined maximum value and anoperating speed of the engine is less than the predetermined engineoperating speed.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the disclosure, reference will be made tothe following detailed description which is to be read in connectionwith the accompanying drawing, wherein:

FIG. 1 shows a schematic diagram of an exemplary embodiment of atransport refrigeration system wherein the compressor is by a motorpowered by a electric generator driven by a diesel engine;

FIG. 2 shows a schematic diagram of an exemplary embodiment of atransport refrigeration system wherein the compressor is driven by adiesel motor through a belt drive; and

FIGS. 3(a) & (b) show a block diagram illustration of an exemplaryembodiment of a control method as disclosed herein.

DETAILED DESCRIPTION

Referring initially to FIGS. 1 and 2 of the drawing, there are depictedexemplary embodiments of transport refrigeration systems for cooling theatmosphere within the cargo box of a truck, trailer, container,intermodal container or similar cargo transport unit. The transportrefrigeration system 10 includes a transport refrigeration unit 12including a compressor 14, a refrigerant condenser heat exchanger 16, anexpansion device 18, a refrigerant evaporator heat exchanger 20 and asuction modulation valve 22 connected in a closed loop refrigerantcircuit including refrigerant lines 24, 26 and 28 and arranged in aconventional refrigeration cycle. The transport refrigeration system 10further includes an electronic system controller 30, a diesel engine 32and an engine controller 34. The transport refrigeration system 10 ismounted as in conventional practice to an exterior wall of the truck,trailer or container with the compressor 14 and the condenser heatexchanger 16 with its associated condenser fan(s) (not shown) and dieselengine 32 disposed externally of the refrigerated cargo box.

As in conventional practice, when the transport refrigerant unit 12 isoperating in a cooling mode, low temperature, low pressure refrigerantvapor is compressed by the compressor 14 to a high pressure, hightemperature refrigerant vapor and passed from the discharge outlet ofthe compressor 14 into refrigerant line 24. The refrigerant circulatesthrough the refrigerant circuit via refrigerant line 24 to and throughthe heat exchange tube coil or tube bank of the condenser heat exchanger16, wherein the refrigerant vapor condenses to a liquid, thence throughthe receiver 36, which provides storage for excess liquid refrigerant,and thence through the subcooler coil 38 of the condenser. The subcooledliquid refrigerant then passes through refrigerant line 24 through afirst refrigerant pass of the refrigerant-to-refrigerant heat exchanger40, and thence traverses the expansion device 18 before passing throughthe evaporator heat exchanger 20. In traversing the expansion device 18,which may be an electronic expansion valve (“EXV”) as depicted in FIG. 1or a mechanical thermostatic expansion valve (“TXV”) as depicted in FIG.2, the liquid refrigerant is expanded to a lower temperature and lowerpressure prior to passing to the evaporator heat exchanger 20.

In flowing through the heat exchange tube coil or tube bank of theevaporator heat exchanger 20, the refrigerant evaporates, and istypically superheated, as it passes in heat exchange relationship returnair drawn from the cargo box passing through the airside pass of theevaporator heat exchanger 20. The refrigerant vapor thence passesthrough refrigerant line 26 to the suction inlet of the compressor 14.In passing through refrigerant line 26, the refrigerant vapor traversesa second refrigerant pass of the refrigerant-to-refrigerant heatexchanger 40 in heat exchange relationship with the liquid refrigerantpassing through the first refrigerant pass thereof. Before entering thesuction inlet of the compressor 14, the refrigerant vapor passes throughthe suction modulation valve 22 disposed in refrigerant line 26downstream with respect to refrigerant flow of therefrigerant-to-refrigerant heat exchanger 40 and upstream with respectto refrigerant flow of the compressor 14. The controller 30 controlsoperation of the suction modulation valve 22 and selectively modulatesthe open flow area through the suction modulation valve 22 so as toregulate the flow of refrigerant passing through the suction modulationvalve to the suction inlet of the compressor 14. By selectively reducingthe open flow area through the suction modulation valve 22, thecontroller 30 can selectively restrict the flow of refrigerant vaporsupplied to the compressor 14, thereby reducing the capacity output ofthe transport refrigeration unit 12 and in tum reducing the power demandimposed on the engine 32.

Air drawn from within the cargo box by the evaporator fan(s) (not shown)associated with the evaporator heat exchanger 20, is passed over theexternal heat transfer surface of the heat exchange tube coil or tubebank of the evaporator heat exchanger 20 and circulated back into theinterior space of the cargo box. The air drawn from the cargo box isreferred to as “return air” and the air circulated back to the cargo boxis referred to as “supply air”. It is to be understood that the term“air” as used herein includes mixtures of air and other gases, such asfor example, but not limited to nitrogen or carbon dioxide, sometimesintroduced into a refrigerated cargo box for transport of perishableproduct such as produce.

Although the particular type of evaporator heat exchanger 20 used is notlimiting of the invention, the evaporator heat exchanger 20 may, forexample, comprise one or more heat exchange tube coils, as depicted inthe drawing, or one or more tube banks formed of a plurality of tubesextending between respective inlet and outlet manifolds. The tubes maybe round tubes or flat tubes and may be finned or unfinned.

The compressor 14 may comprise a single-stage or multiple-stagecompressor such as, for example, a reciprocating compressor as depictedin the exemplary embodiments shown in FIGS. 1 and 2. However, thecompressor 14 may be a scroll compressor or other type of compressor asthe particular type of compressor used is not germane to or limiting ofthe invention. In the exemplary embodiment depicted in FIG. 1, thecompressor 14 comprises a reciprocating compressor having a compressingmechanism, an internal electric compressor motor and an interconnectingdrive shaft that are all sealed within a common housing of thecompressor 14. The diesel engine 32 drives an electric generator 42 thatgenerates electrical power for driving the compressor motor which inturn drives the compression mechanism of the compressor 14. The driveshaft of the diesel engine drives the generator shaft. In anelectrically powered embodiment of the transport refrigeration unit 10,the generator 42 comprises a single on-board engine driven synchronousgenerator configured to selectively produce at least one AC voltage atone or more frequencies. In the embodiment depicted in FIG. 2, thecompressor 14 comprises a reciprocating compressor having a compressingmechanism having shaft driven directly by the drive shaft of the dieselengine 32, either through a direct mechanical coupling or through a beltdrive 38 as illustrated in FIG. 2.

As noted previously, the transport refrigeration system 10 also includesan electronic controller 30 that is configured to operate the transportrefrigeration unit 12 to maintain a predetermined thermal environmentwithin the interior space defined within the cargo box wherein theproduct is stored during transport. The controller 30 maintains thepredetermined thermal environment selectively powering the variouscomponents of the refrigerant vapor compression system, including thecompressor 14, the condenser fan(s) associated with the condenser heatexchanger 16, the evaporator fan(s) associated with the evaporator heatexchanger 20, and various valves in the refrigerant circuit, includingbut not limited to the suction modulation valve 22. The controller 30also controls the operation of the compressor 14 to selectively varyingthe output capacity of the compressor 14 to match the cooling demand tomaintain the desired product storage temperature for the particularproducts stored within the refrigerated cargo box.

In one embodiment, the electronic controller 30 includes amicroprocessor and an associated memory. The memory of the controller 30may be programmed to contain preselected operator or owner desiredvalues for various operating parameters within the system. Thecontroller 30 may include a microprocessor board that includes themicroprocessor, an associated memory, and an input/output board thatcontains an analog-to-digital converter which receives temperatureinputs and pressure inputs from a plurality of sensors located atvarious points throughout the refrigerant circuit and the refrigeratedcargo box, current inputs, voltage inputs, and humidity levels. Theinput/output board may also include drive circuits or field effecttransistors and relays which receive signals or current from thecontroller 30 and in turn control various external or peripheral devicesassociated with the transport refrigeration system. The particular typeand design of the electronic controller 30 is within the discretion ofone of ordinary skill in the art to select and is not limiting of theinvention.

The system controller 30 is also in communication with the electronicengine controller 34. For example, the system controller 30 may be inclosed loop communication with the electronic engine controller 34 byway of a controller area network (CAN) system. The system controller 30determines the operating load state of the engine 32 based on inputreceived from the electronic engine controller 34. For example, in anembodiment, the electronic engine controller 34 senses the position ofthe mechanical fuel rack, which essentially represents a fuel throttleposition, and is indicative of the level of fuel flow being supplied tothe engine 32 relative to the maximum permissible fuel flow fuel, whichis indicative of the operating engine load relative to the maximumoperating engine load. The electronic engine controller 34 also sensesthe operating engine speed, that is engine revolutions per minute (RPM),of the engine 32 in real time. The system controller 30 monitors boththe fuel rack position and the operating engine speed throughinterrogation of the electronic engine controller 34. For example, in anembodiment, the electronic engine controller 34 may detect the fuel rackposition and the operating engine speed (RPM) at one second intervals,and the system controller 30 may determine engine load based on arunning average of the past thirty seconds of readings for both fuelrack position. Engine operating speed (RPM) may also be based on arunning average of RPM measurements taken over a time interval, forexample the past thirty seconds.

In accordance with an aspect of the disclosure, the system controller 30optimizes the performance of the transport refrigeration system 10 bymatching the capacity output of the transport refrigeration unit 12 toan available shaft power of the diesel engine, which equates to matchingthe power demand of the refrigerant unit 12 to an available shafthorsepower of the diesel engine 32. By doing so, enhanced fuel economyand improved system capacity control can be realized. Additionally,engine output can be maximized while avoiding engine overload and enginestalls. Controlling the power consumption of the transport refrigerationsystem can also permit a smaller engine, that is an engine having alower maximum available shaft power, to be used. By monitoring both thefuel rack position and the operating engine speed, the system controllerdetermines the real time operating load state of the engine 32 and canadjust the capacity output of the transport refrigeration unit 12 tomatch the available shaft power of the engine 32 as necessary. Forexample, the system controller 30 can adjust the capacity output of thetransport refrigeration unit 12 by selectively adjusting the suctionmodulation valve (SMV) 22 to adjust the flow of refrigerant vapor to thesuction inlet of the compressor 14. The system controller 30 can alsoadjust the capacity output of the transport refrigeration unit 12 byother techniques known in the art such as, but not limited to, unloadingthe compressor 14 to reduce the flow of high pressure refrigerantthrough the refrigerant circuit, ceasing operation in an economizermode, throttling the evaporator expansion valve closed or a combinationthereof.

In determining the operating state of the engine 32, the systemcontroller 30 analysis the operating fuel rack position, expressed as apercent of the fuel rack position at a 100% fuel flow setting, and theoperating engine speed in RPM, expressed as a percent of the targetengine RPM, which is an indication of engine RPM droop, that is adrop-off in the real-time operating engine RPM relative to the targetengine RPM. Thus, as used herein, an engine RPM droop of 98% would meanthat the operating engine RPM is two percent below the target engineRPM. Similarly, an engine RPM droop of greater than 98% would indicatean operating engine RPM that is less than 2% below the target engine RPMand an engine RPM droop of less than 98% would indicate an operatingengine RPM that is more than 2% below the engine target RPM. In manyapplications, the diesel engine 32 may have two operating RPM points,that is a relatively lower RPM for low speed operation and a relativelyhigher RPM for high speed operation. In such case, the target RPM wouldbe selected by the system controller 30 based upon whether the engine 32was currently operating in a low speed mode or a high speed mode.

In an aspect of the method of optimizing the performance of thetransport refrigeration system as disclosed herein, the systemcontroller 30 operates the transport refrigeration system in a normaloperating mode at a capacity output necessary to meet a currentrefrigeration demand load so as long as the operating fuel rack positionis not at 100% with the engine operating speed dropping no more than afew percent, for example dropping no more than two percent (i.e. anengine RPM droop of no less than 98%). An operating engine RPM of lessthan 98% of the target engine RPM could indicate an impending enginestall condition. In the normal operating mode, the system controller 30will permit engine speed shifts, changes in unloader state (on/off),rapid opening or closing of the suction modulation valve 22, and othernormal operations. However, when the fuel rack position reaches orexceeds 90% and the operating engine RPM simultaneously drops to 98% ofthe target engine RPM, the controller 30 unload the compressor 14 and/orclose the suction modulation valve 22 to reduce engine load and returnthe fuel rack position to less than 85% and raise the operating engineRPM to greater than 98% of the target engine RPM. Under these conditionsthe system controller 30 will allow the rate at which the SMV may befurther closed to reach its maximum closing speed. Once the engine loadhas been reduced, so long as a system change request is not active, thesystem controller 30 will limit the maximum opening speed of the suctionmodulation valve (SMV) 32, that is the rate at which the SMV may befurther opened, to 0.1% per second and employ as a control limit logicmaintaining the fuel rack position at less than 90% and maintaining theengine operating RPM equal to or greater than 98% of the engine targetspeed.

If under these conditions wherein the fuel rack position reaches orexceeds 90% and the operating engine RPM simultaneously drops to 98% ofthe target engine RPM, system change is called for, for example a speedshift or a change in unloader state, the system controller 30 willunload the compressor 14 and/or close the suction modulation valve (SMV)22 to further reduce the engine load to bring the fuel rack position toless than 70% and raise the operating engine RPM to greater than 98% ofthe target engine RPM. The system controller 30 will also engage controllimit logic and bring the engine operating state to a fuel rack positionof less than 90% and maintain the operating engine RPM at least 98% ofthe target RPM, thereby ensuring against an engine stall or engineoverload condition occurring. Once the engine load has been reduced, thesystem controller 30 will allow the system change request but will limitthe maximum opening speed of the suction modulation valve (SMV) 32, thatis the rate at which the SMV may be further opened, to 0.1% per secondand employ as a control limit logic maintaining the fuel rack positionat less than 90% and maintaining the engine operating RPM equal to orgreater than 98% of the engine target speed.

Referring now to FIG. 3, there is depicted in a process schematic blockdiagram illustrating an exemplary embodiment of a method of thedisclosure. At block 300, the controller 30 initiates the process bydetermining at step 302 both the operating fuel rack position (Rack) asa percent of the maximum fuel rack position at 100% fuel flow to theengine 32 and the operating engine speed as a percent of the maximumengine speed (RPM droop). Both determinations are made based on realtime engine operating data obtained from the electronic enginecontroller 34. As explained earlier, both determinations may be runningaverages over a selected time interval, such as, for example, theaverage over a 30 second running period of individual readings made atone second intervals during that period. At block 302, a determinationis made as to whether the operating fuel rack position (Rack) is lessthan 85% and whether the operating engine speed (RPM) is at least 98% ofthe target engine operating speed (RPM droop). If yes to bothconditions, i.e. the operating fuel rack position (Rack) is less than85% and the operating engine speed is equal to or greater than 98% ofthe target engine operating speed (RPM droop), the controller 30maintains normal operation (block 318) of the transport refrigerationsystem 12.

However, if either one or both of the conditions at block 302 is no,then the system controller 30, at block 304, determines whether theoperating fuel rack position (Rack) is at or greater than 90% andwhether the operating engine speed is less than 98% of the target engineoperating speed (RPM droop). If no to both conditions, i.e. theoperating fuel rack position (Rack) is less than 90% and the operatingengine speed is at least 98% of the target engine operating speed (RPMdroop), the controller 30 maintains normal operation (block 318) of thetransport refrigeration system 12.

If, at block 304, the controller 30 determines that the operating fuelrack position (RACK) is at or greater than 90% and the operating enginespeed is less than 98% of the target engine operating speed (RPM droop),the system controller 30, at block 306, will reduce the cooling outputcapacity of the refrigeration unit 12, for example by initiating anunload of the compressor 14 or closing the suction modulation valve(SMV) 22, to bring the operating fuel rack position (Rack) to less than85% and the operating engine speed to at least 98% of the target engineoperating speed (RPM droop). Next, at block 308, the system controller30 checks to determine whether a system change request is active. Asystem change request could be, for example, for purposes ofillustration but not limitation, a change in engine speed, a compressorunload, or transient condition. If a change request is not active atblock 308, the system controller 30 proceeds directly to block 312, andrestricts the maximum rate of opening of the suction modulation valve(SMV) 22. to 0.1 percent per second, thereby limiting the rate ofincrease in refrigerant vapor flow to the suction inlet of thecompressor 14, which in turn limits a change in capacity output of thetransport refrigeration unit 12. At block 314, the system controller 30now implements as its control limit logic maintaining the operating fuelrack position (Rack) to less than 90% and the operating engine speed atleast 98% of the target engine operating speed (RPM droop). The systemcontroller 30 will continue to monitor, block 316, the operating fuelrack position (RACK) and the operating engine speed, and permit normaloperation (block 318) of the refrigeration system, including therefrigeration unit 12, so long as both the operating fuel rack position(Rack) remains less than 90% and the operating engine speed is at least98% of the target engine operating speed (RPM droop).

However, if a system change request is active at block 308, the systemcontroller 30, at block 310, will reduce the cooling output capacity ofthe refrigeration unit 12, for example by initiating an unload of thecompressor 14 or closing the suction modulation valve (SMV) 22, to bringthe operating fuel rack position (Rack) to less than 70% and theoperating engine speed to at least 98% of the target engine operatingspeed (RPM droop). The system controller 30 then proceeds to block 312and restricts the maximum rate of opening of the suction modulationvalve (SMV) 22. to 0.1 percent per second, thereby limiting the rate ofincrease in refrigerant vapor flow to the suction inlet of thecompressor 14, which in tum limits a change in capacity output of thetransport refrigeration unit 12. At block 314, the system controller 30now implements as its control limit logic maintaining the operating fuelrack position (Rack) to less than 90% and the operating engine speed atleast 98% of the target engine operating speed (RPM droop). During theperiod of operation of the refrigerant unit at reduced cooling capacityand with restriction on the rate of opening of the suction modulationvalve (SMV) 22, the system controller 30 will allow the requested systemchange occur with little or no risk of an engine overload or enginestall occurring that would lead to insufficient shaft horsepower outputfrom the engine. Thus, the risk of a shutdown of the refrigerant system14 occurring as a result of the system change being implemented isgreatly reduced, if not eliminated.

Additionally, at block 316, the system controller 30 continues tomonitor whether the operating fuel rack position (Rack) is less than 90%and whether the operating engine speed (RPM) is at least 98% of thetarget engine operating speed (RPM droop). If, at block 316, the systemcontroller 30 determines that the operating engine speed drops below 98%of the target operating engine speed (RPM droop) or the operating fuelrack position rises above 90%, a system alarm is activated, at block320, to warn of a potential impending engine stall or engine overload.

The terminology used herein is for the purpose of description, notlimitation. Specific structural and functional details disclosed hereinare not to be interpreted as limiting, but merely as basis for teachingone skilled in the art to employ the present invention. Those skilled inthe art will also recognize the equivalents that may be substituted forelements described with reference to the exemplary embodiments disclosedherein without departing from the scope of the present invention.

While the present invention has been particularly shown and describedwith reference to the exemplary embodiments as illustrated in thedrawing, it will be recognized by those skilled in the art that variousmodifications may be made without departing from the spirit and scope ofthe invention. For example, in other embodiments, a different indicatorof operating engine load, other than fuel rack position, could be usedto monitor the operating engine load, in combination with operatingengine speed in carrying out the concept of the method disclosed herein.

Therefore, it is intended that the present disclosure not be limited tothe particular embodiment(s) disclosed as, but that the disclosure willinclude all embodiments falling within the scope of the appended claims.

What is claimed is:
 1. A method for controlling the performance of atransport refrigeration system having a transport refrigeration unitpowered by an engine, comprising matching a capacity output of thetransport refrigeration unit to an available shaft power of the engine;the method further comprising: monitoring an operating engine load ofthe engine; monitoring an operating engine speed of the engine;operating the transport refrigeration system at the capacity outputnecessary to meet a current refrigeration demand load so as long as boththe operating engine load of the engine is not at or above apredetermined maximum value and the operating engine speed of the engineis not below a predetermined engine operating speed; and reducing thecooling capacity output of the refrigeration unit when the operatingengine load of the engine is at or above the predetermined maximum valueand an operating engine speed of the engine is less than thepredetermined engine operating speed.
 2. The method of claim 1, furthercomprising operating the transport refrigeration system at the capacityoutput necessary to meet the current refrigeration demand load so aslong as both the operating engine load of the engine is not at a secondpredetermined maximum value and the operating engine speed of the engineis not below a second predetermined engine operating speed.
 3. Themethod of claim 1, wherein the predetermined maximum value is 100%. 4.The method of claim 1, wherein the predetermined maximum value is 90%.5. The method of claim 1, wherein the predetermined operating speed is98% of a target engine operating speed.
 6. The method of claim 2,wherein the second predetermined maximum value is 85%.
 7. The method ofclaim 2, wherein the second predetermined operating speed is 98% of atarget engine operating speed.
 8. The method of claim 2, wherein thepredetermined maximum value and the second predetermined maximum valueare the same.
 9. The method of claim 2, wherein the predeterminedmaximum value and the second predetermined maximum value are different.10. The method of claim 2, wherein the predetermined engine operatingspeed and the second predetermined engine operating speed are the same.11. The method of claim 2, wherein the predetermined engine operatingspeed and the second predetermined engine operating speed are different.12. The method of claim 1 wherein the transport refrigeration unitincludes a compressor driven by the engine.
 13. The method of claim 12wherein the engine directly drives the compressor.
 14. The method ofclaim 12 wherein the engine drives an electric generator for generatingelectrical power to the compressor.
 15. The method of claim 1 whereinthe engine is a diesel engine.
 16. The method of claim 1 wherein thepredetermined engine operating speed comprises a predeterminedpercentage of a target engine operating speed.
 17. The method of claim 1wherein the operating engine load of the engine is indicated by a fuelflow.
 18. The method of claim 1 wherein the operating engine speed ofthe engine is indicated by RPM.
 19. The method of claim 1 whereinreducing the cooling capacity output of the refrigeration unit comprisesadjusting an opening of a suction modulation valve to adjust a flow ofrefrigerant to a suction inlet of a compressor.